To survey clients at a massage-therapy spa, a researcher alphabetically lists all of the customers the spa had last month. The researcher sends a survey to the 13th person on the list (with the number 13 being randomly chosen), then surveys every 20th customer after that. Which sampling method was used?

The researcher chooses a random starting point—the 13th person—then selects every 20th customer from the alphabetical list. Selecting people at a regular interval after a random start is systematic random sampling, so the correct choice is systematic random sampling.

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